This application is based on the European Patent Application No. 97830674.4, the content of which is incorporated hereinto by reference.
I. Field of the Invention
The present invention relates to an optical code scanning reader with laser beams travelling along a minimum of two different optical paths and focused to read said optical code at different distances.
II. Related Art and other Considerations
Patent EP-0 480 348 describes a bar code reader comprising a minimum of two laser beam sources and suitable optical means for guiding the emitted beams onto a reading area and receiving the light diffused from said area. The optical means focus each beam independently on different areas of said reading area, thus enabling an optical code to be read at different reading distances.
Each of the laser beams is capable of xe2x80x9cresolvingxe2x80x9d a portion of a bar code having a minimum assigned dimension in a given area around the focal point and said area determines the depth of field of the reader for said portion of code.
The above-mentioned reader uses two laser beams focused on two different points. One beam, dedicated to reading the optical code from a close distance, is focused on a point closer to the optical receiving means than the focal point of the other beam which is dedicated to reading from a remote distance. Two reading fields are thus obtained which, although partially overlapping, create a total depth of field greater than that of a single laser beam.
In the reader in accordance with EP-0 480 348 the laser beams travel along xe2x80x9ccoincidentxe2x80x9d optical paths and the individual optical paths are caused to overlap by the use of deflector elements consisting of beamsplitters. The beamsplitters, however, reduce the utilisable power of each individual laser beam by 50%. Where several laser beams are present, therefore, loss of power increases in proportion to the number of beamsplitters used, reducing the available power by 50% for each additional beamsplitter.
The reader described in the above-mentioned patent could hypothetically be improved by reducing the loss of available power by means of polarized beamsplitters, since it is well-known that these are more efficient.
However, if wishing to use more than two lasers, the subsequent beamsplitters would in any case have to be of non-polarized type and this would, as already mentioned, lead to a loss of power of 50% for each conventional type of beamsplitter added.
Polarized beamsplitters could moreover not be used when optical codes printed on shiny surfaces have to be read. In such cases it is in fact necessary for the vector of polarization, by means of which the laser beams are guided onto the code, to be identically orientated.
The inventors took into consideration the possibility of causing the different laser beams to travel along different optical paths, but this gives rise to the problem of parallax. In fact, except for one laser beam, all the other beams are offset in respect to the optical axis of the optical receiving means, i.e. said other beams occupy a spatial position remote from the optical axis. Since offset between laser beam and optical receiving means causes a shift of the image of the optical code which is formed on a photoreceiver (the parallax phenomenon), it is possible for a significant part of the light diffused by the optical code to be directed outside the sensitive area of the photoreceiver.
The inventors have, however, found that this problem can be minimised by suitably offsetting the laser beams in such a way that their distance from the optical axis of said optical receiving means is greater for the laser beam which reads the optical code at the maximum distance and is gradually reduced for the laser beams which read the optical code at gradually decreasing distances.
The inventors have also found that optimum reading of the optical code can be obtained by selection of a minimum reading distance such that at least a prefixed quantity of energy of the light diffused by said optical code falls within the sensitive area of the photoreceiver.
In accordance with the present invention, an optical code scanning reader comprising means for generating laser beams, means for directing the laser beams onto an optical code to be read, least one optical receiving means for collecting and focusing the light diffused by said optical code on a photoreceiver element associated with said optical receiving means, said optical receiving means having an its own optical axis, characterized in that said laser beams travel along at least two different optical paths and in that at least one of said optical paths is at a distance Ho from said optical axis.
In accordance with a preferred embodiment, the reader also comprises means of focusing said laser beams at different reading distances from said optical receiving means, said means defining, for each of said laser beams, a respective depth of reading field.
Advantageously, said distance (Ho) has values which decrease with diminishing reading distance. Furthermore, the quantity of energy which reaches said photosensitive area for each beam, in the worst reading condition, must be greater than a preselected minimum.
Preferably, said distance (Ho) is maximum for the beam which reads said optical code at the maximum distance (somax), whilst the distance is minimum for the beam which reads said optical code at the minimum distance (somin).
More preferably, said distance (H0) is zero for the beam which reads said optical code at the minimum distance (somin), said beam being coaxial to said optical axis of said optical receiving means.
Even more preferably, said distance (Ho) has values intermediate between said maximum and minimum distances for any beams which reads said optical code at intermediate distances.
Advantageously, said laser beams have depths of field which present overlapping areas.
In accordance with another preferred embodiment, each one of said laser beams is generated by a single laser source.
Preferably, each of said laser beams is obtained by the combination, substantially coaxial, of at least two of the laser sources focused at different distances, said combination being obtained by combinatory means. Said combinatory means may be beamsplitters or polarized beamsplitters.
More preferably, the maximum distance (Ho) of said laser beams, in relation to the distance (so) along the optical axis of said optical receiving means, is selected in such a way that the barycentre of the lightspot obtained by focusing of the light diffused by said code by means of said optical receiving means falls within the sensitive area of the photoreceiver element.
In particular, said maximum distance (H0) is selected by means of the following relation:
Ho less than Hpd*so/spd
where
so is the reading distance,
Hpd is the maximum semi-dimension of the photoreceiver element, and
spd represents the distance of the photoreceiver element from the optical receiving means.
Preferably, the maximum distance (Ho) is selected by means of the following relation:
Ho less than Hpd*so(1/fxe2x88x921/somax)
where
Hpd is the maximum semi-dimension of the photoreceiver element, f is the focal length of said optical receiving means, and somax is the maximum reading distance of said optical code.
Advantageously, said photoreceiver element is positioned at a distance (spd) from said optical receiving means such that the light diffused by said optical code, at the maximum reading distance (somax) is substantially focused on said photosensitive area of said photoreceiver.
Preferably, the minimum reading distance (somin) of said optical code by said optical beams is selected in such a way that the quantity of energy of the lightspot, obtained by focusing of the light diffused by said optical code by means of said optical receiving means, falling within the photosensitive area of said photoreceiver element is at least equal to the quantity of energy of the light falling on said photosensitive area in the reading conditions at the maximum distance (somax).
Advantageously, the minimum reading distance (somin) of said optical code is selected by means of the following relation:       s    omin    =                    s        pd            -              2        ⁢                  H          pd                *                              s            omax                    /                      D            rx                                                        s          pd                /        f            -      1      
where
Drx is the diameter of said optical receiving means, and
f is the focal length of said optical receiving means.
Furthermore, the above-mentioned optical paths of said laser beams may be substantially parallel to said optical receiving means or substantially convergent.
Preferably, at least two of said optical paths of said laser beams intersect each other in an overlapping area of the respective depths of field.
In accordance with a further embodiment, the reader also comprises a rotor having mirror faces and an axis of rotation, said rotor being associated with said focusing means and said optical receiving means.
In particular, the above-mentioned laser beams may be aligned on a plane parallel or orthogonal to said axis of rotation of said rotor.
The reader in accordance with the invention therefore enables the required overall depth of field to be obtained without detrimental reductions of power of the laser beams.